Commercial Nematic Liquid Crystal Research Articles

Low-Temperature Metallomesogen Model Structures and Mixtures as Potential Materials for Application in Commercial Liquid Crystal Devices

The present work was the preliminary study of phase diagrams and miscibilities of low-temperature metallomesogen (MOM) model structures based on rod-like palladium (Pd) alkyl/alkoxy-azobenzene metal complexes and their mixtures with commercial liquid crystal materials for potential application. The initial results indicated the accessible temperature range and mesgenic miscibility between parent ligand, MOMs and commercial liquid crystal mixtures. The eutectic ligand/MOM composition with other MOMs and commercial nematic liquid crystal materials exhibited complete mesogenic miscibility and wide low-temperature mesogenic stability for eventual utilization in commercial liquid crystal devices.

Metallomesogens as potential materials for improvement of electro-optical properties of commercial liquid crystals

We present a new approach to utilize the eutectic metallomesogen (MOM) mixtures to improve the electro-optical properties of commercial nematic liquid crystal (NLC) materials. Accordingly, we studied the phase diagrams of rod-like MOM models structures, including alkyl/alkoxy-azobenzene with Pd complex and salicylal-diaminates with Cu, Ni and Pd complexes. These phase diagrams we utilized the eutectic MOM mixtures and mixed them with few commercial NLC hosts. The results showed a complete mesogenic miscibility and wide mesogenic range. Although the studied MOM model materials are not the optimum structures and we did not study the electro-optical properties of MOM-NLC mixtures, the present results indicate the potential application of MOMs to improve the performance of commercial NLC, which require further future studies.

Induced Red Circularly Polarized Luminescence Emission Promoted by Intermolecular Förster Resonance Energy Transfer through a Cholesteric Liquid Crystal Medium

In this paper, planar chiral helicenes based on [2.2]paracyclophane PCPH5 acted as both chiral inducers and energy donors to construct the circularly polarized luminescence (CPL)-active ternary cholesteric liquid crystals (T-N*-LCs) in a commercial nematic liquid crystal (SLC1717, N-LCs) medium. Induced red CPL emission based on the energy acceptor of the achiral polymer DTBTF8 could be successfully promoted by the intermolecular Förster resonance energy transfer mechanism. The resulting T-N*-LCs can generate intensive CPL signals with glum up to +0.70/-0.67. Interestingly, this kind of "on-off" CPL switching of T-N*-LCs can be manipulated by the applied direct current electric field.

Nanotechnology for purifying nematic liquid crystals based on magnetic separation accompanied by phase transition

Free ions are generally unfavorable in liquid crystal (LC) displays, and LC purification technologies are critically important. The colloidal γ-Fe2O3 magnetic nanoparticles coated with oleic acid (γ-Fe2O3@OA MNPs) have a high ratio of surface to volume, which may adsorb more free ions and are uniform in the LC at room temperature. In this work, the precipitation and separation of the doped colloidal γ-Fe2O3@OA MNPs resulting from the magnetic field accompanied by an isotropic-nematic phase transition are more efficient than in the single case of the phase transition or the magnetic field. The residual ion concentrations have decreased distinctly using the low gradient magnetic field (∇ B ∼ 2 T/m) with the phase transition. In addition, when the doped colloidal γ-Fe2O3@OA MNPs are 0.4 % and 0.2 % by weight, the former concentrations of the residual ions and γ-Fe2O3@OA MNPs are lower than the latter. As a result, the commercial nematic LC can be purified by this approach based on nanotechnology in our study.

Carbon Nanotubes Blended Nematic Liquid Crystal for Display and Electro-Optical Applications

In this paper, we investigate a commercial nematic liquid crystal (LC) mixture namely E7 dispersed with small concentrations of multi-walled carbon nanotubes (MWCNTs). The dielectric and electro-optical characterizations have been carried out in the homogeneously and vertically aligned LC cells. The electro-optical response of LC molecules has been enhanced by 60% after the addition of MWCNTs, which is attributed to the reduced rotational viscosity in the composites. MWCNTs act like barricades for ionic impurities by reducing them up to ∼34.3% within the dispersion limit of 0.05 wt%. The nematic–isotropic phase transition temperature (TNI) of the E7 LC has also been shifted towards the higher temperature, resulting in a more ordered nematic phase. The enhanced birefringence and orientational order parameter in the LC-MWCNTs are attributed to π-π electron stacking between the LC molecules and the MWCNTs. The outlined merits of the LC-MWCNTs composites evince their suitability for ultrafast nematic-based electro-optical devices.

Dielectric and electrooptical properties of hockey-stick-shaped liquid crystal with a negative dielectric anisotropy

We investigated dielectric properties of a hockey-stick-shaped liquid crystal (HLC). Two dielectric relaxation modes were observed at 0.91 kHz and 4.51 MHz. The low frequency relaxation modes in isotropic, smectic, and nematic phases are related to the motion of ions, collective tilt fluctuation, and rotation around the long molecular axis, respectively. Meanwhile, the high frequency relaxation modes in nematic and smectic phases was due to the rotation around the short axis of the molecules and hindered by the resistance of the ITO layers. We also examined the electrooptical response of the 5.0 wt% HLC-doped commercial nematic liquid crystal (LC) mixtures. The birefringence of the LC mixture was slightly increased, while the falling time and the rotational viscosity was decreased in the nematic phase.

Ultrastrong Red Circularly Polarized Luminescence Promoted from Chiral Transfer and Intermolecular Förster Resonance Energy Transfer in Ternary Chiral Emissive Nematic Liquid Crystals

Chiral emissive liquid crystals (N*-LCs) have been proved to greatly amplify the circularly polarized luminescence (CPL) signals due to highly regular spiral arrangement of dyes in a well-organized liquid crystals system. Normally, CPL materials with a high luminescence dissymmetry factor (glum) and quantum yield (QY) can meet the real application requirement. Here, four chiral aggregate-induced emission (AIE) active donors (Guests A1-A4: R-C2, R-C4, R-C6, R-C8, chiral dopant, and energy donor) and achiral AIE-active acceptors (Guest B: PBCy, CPL emitter) were doped into the commercial nematic liquid crystals E7 (N-LCs, Host) to form CPL-active ternary chiral emissive N-LCs (T-N*-LCs), respectively. This kind of T-N*-LCs could emit strong red CPL with QY = 16.56% and glum up to 1.51 through intermolecular energy transfer and chirality induction from the supramolecular self-assembly of T-N*-LCs. This work provides the effective strategy for the development of high glum CPL materials.

Enhancement of flexoelastic ratio of nematic liquid crystal doped with hydrogen-bonded bimesogen molecules

We synthesized a new liquid crystal (LC) dopant 4-[7-(4′-Cyanobiphenyl-4-yl)hexyloxy]bezoic acid (CBO7OBA) which can form dimers by hydrogen-bonding. We mixed the hydrogen-bondable CBO7OBA dopants with commercial nematic liquid crystal (NLC) mixtures and investigated the change of the flexoelastic ratio of the mixture. The flexoelastic ratio of the CBO7OBA-doped NLC mixture was increased up to 4.7 C/Nm, while that of the pure NLC was 2.0 C/Nm at room temperature. The greater flexoelastic ratio of the CBO7OBA-doped NLC mixture is thought to be due to the bent-shape and the large dipole moment of the CBO7OBA molecules.

Spontaneous homeotropic alignment of nematic liquid crystals induced by a double-armed side chain liquid crystalline polymethacrylate

ABSTRACTA liquid crystalline polymethacrylate having two cyclohexylphenyl mesogens on its side-chain (PMG0) was synthesized by the radical polymerization of the corresponding methacrylate monomer (MG0). PMG0 exhibited a smectic A phase around room temperature showing a focalconic fan texture in POM observations and a broad diffusion in a wide angle range of its WAXD profile. Upon slow cooling at −1 °C min−1 from the isotropic melt, PMG0 exhibited spontaneous homeotropic alignment between two native glass surfaces. A mixture of the commercial nematic liquid crystal (ZLI-4792) containing 1.0 wt% PMG0 also exhibited a homeotropic alignment, which easily covered hydrophilic surfaces such as glass or polyacrylamide.

Induced cholesteric solutions of highly soluble bis-salicylaldiminate Cu(II) metallomesogens and their rheological behaviour

Copper(II) complexes containing chiral centres and functionalised with citronellyl end groups were synthesised. Isomerisation of citronellyl double bond was exploited to decrease the melting temperature of the complexes and improve their solubility in organic media. The chiral metallomesogens were successfully used to produce induced cholesteric solutions in commercial nematic liquid crystals also in high concentrations. Transparent and coloured thin films were easily obtained showing good reflective properties across the visible spectrum. The rheological behaviour of the nematic and the cholesteric phases was investigated. The nematic system showed the characteristics of a tumbling phase with a very peculiar transient behaviour. The cholesteric samples also showed a very unusual, flow-history-dependent transient behaviour. The viscosity trend presumably indicates a flow-induced orientation of the cholesteric superstructure, thus opening the way to the possibility of tuning the yield stress level of the material. The results support the potentiality of these compounds for applications in light reflecting technology.

Asymmetric Director Structures and Ions in the Measurement of the Flexoelectric Sum (e1 + e3)

Hybrid Aligned Nematic (HAN) devices filled with highly ionic liquid crystal materials screen out the internal bias due to surface polarization, making it possible to determine the flexoelectric sum, e1 + e3. A transient change is observed in the device response due to differing polarities of an external offset applied to an AC signal, with a time constant similar to the characteristic ion relaxation time in the material. Assuming ions screen out the internal bias, data-theory fits in the low frequency, low voltage perturbative regime gives a value of /verte1 + e3/vert = 10 pCm−1 for the standard commercial eutectic E70 nematic liquid crystal mixture.

Azo containing thiophene based prop-2-enoates for photoalignment of a nematic liquid crystal

The ability of suitably derived prop-2-enoates from 3-(thiophen-2-yl)- and 3-(thiophen-3-yl)-prop-2-enoic acid and appropriately fluorinated and non-fluorinated 4-[(4-alkoxyphenyl)diazenyl]phenols to promote excellent photoalignment of a bulk commercial nematic liquid crystal is reported. The performed study showed that the photoalignment of the liquid crystal is influenced by the number and disposition of fluoro-substituents and the position of attachment of the terminal thiophene moiety, either 2- or 3-substituted, in the molecular structure of the alignment material. Inclusion of a lateral fluoro-substituent(s) induces an excellent photoalignment whereas the non-fluorinated counterpart tends to give an unsatisfactory alignment. 3-Substituted thiophenes give a better photoalignment than their 2-substituted isomeric counterparts. A prototype display, which contains an alignment layer made from one of the proposed materials, possessing good alignment ability, is presented in order to demonstrate the application potential of these materials in LCDs.

Mesogenic BODIPYs: an investigation of the correlation between liquid crystalline behaviour and fluorescence intensity

A series of mesogenic molecules based on the 4,4'-difluoro-3a,4a-diaza-s-indacene (BODIPY) fluorophore have been synthesised and characterised. Each compound consists of one mesogenic unit, based on a cyanobiphenyl core, and one fluorophore, of varying alkyl substitution. The compounds were prepared by microwave-assisted palladium-catalyzed couplings (Suzuki and Sonogashira) due to slow reaction rates under conventional heating conditions. The effect of increasing the molecular length was also investigated by incorporating an ethynyl unit between the mesogen and the fluorophore. The molecules self-assemble into monotropic nematic phases which were identified by optical polarising microscopy (OPM) and differential scanning calorimetry (DSC). The compounds were found to display varying degrees of fluorescence quantum yield dependant on the number of alkyl substituents attached to the fluorophore. A relationship was observed between the nematic phase stability and fluorescence intensity of the compounds with increasing alkyl substitution causing an increase in fluorescence intensity due to restriction of the 8-phenyl ring rotation while simultaneously disrupting nematic phase formation, causing a reduction in nematic range. Temperature-dependant fluorescence measurements were also acquired along with fluorescence measurements of four of the synthesised BODIPYs dissolved in a commercial nematic liquid crystal (BL024) and incorporated into a twisted nematic cell.

Determination of the molecular tilt profile of a liquid crystal under applied electric field by generalized transmission ellipsometry

It has been demonstrated that, with the use of a standard rotating analyzer ellipsometer, generalized transmission ellipsometry (GTE) is able to uniquely determine the molecular tilt profile of a commercial nematic liquid crystal variable retarder (LCVR) with applied voltage. Also, the presence of the tilted anisotropic layer permits us to determine, solely with GTE measurements, both the ordinary and extraordinary indices of refraction of the liquid crystal. The isotropic layers of the LCVR, which are of no interest in this kind of measurement, can be completely omitted during modeling. The sensitivities of the technique to the determination of the ordinary index of refraction, the difference between ordinary and extraordinary indices of refraction, and tilt profile are discussed with strategies to improve the three. Previous measurements using guided modes that require special LCVR cells, or the complete modelling of the device due to the use of transmission and reflection data, are in agreement with the results obtained in this paper.

Deformation of isotropic and anisotropic liquid droplets dispersed in a cellulose liquid crystalline derivative

In this work we study the strain-induced deformation of both isotropic and anisotropic liquid droplets dispersed in a liquid crystalline cellulose matrix. We have produced two types of acetoxypropylcellulose (APC) solid films one with a droplet dispersion of the commercial liquid crystal E7 from Merck, and another with a droplet dispersion of silicone oil. To produce the solid films a solution of APC (60%wt) in dimethylacetamide (DMAc) with 15%wt of either the commercial nematic liquid crystal E7 or the silicone oil was prepared. After homogenization the phase separated solutions were submitted to a shear flow mechanical field and casted onto a Teflon plate. We performed mechanical uniaxial stress-strain tests in the free standing films recording continuously the strain and images of the deformed droplets. The mechanical tests were carried out using a mini stress-strain testing machine apparatus and an Olympus optical polarizing microscope with an attached CCD camera. The images obtained from the mechanical tests for each value of the imposed strain were then analyzed comparing the images of deformed droplets with those of the undeformed ones, extracting in this way the local strain field. The droplet deformation data obtained show, as expected, significant differences in the local strain field when stretching parallel and perpendicular to the initial shear direction. No significant differences were found in the local strain fields obtained from the silicone oil and the E7 droplets films. The local strain fields variation with the imposed strain are compared with the predictions of the theory developed for nematic elastomers by Warner and Terentjev (Liquid crystal elastomers. Clarendon Oxford Press, Oxford, 2003).

Metallomesogens as Biaxial Dopants in a Calamitic Nematic Liquid Crystal

The electric field induced nematic order reconstruction is a way to achieve controlled transitions between uniaxial nematic textures with different topologies, hence allowing intrinsic textural bistability. The order reconstruction connects perpendicular nematic director orientations by exchanging two eigenvalues of the nematic order tensor Q , implying intermediate transient biaxial order. This phenomenon avoids the full nematic melting; in fact the nematic scalar order parameter does not vanish. The Biaxial Order Reconstruction in a calamitic Nematic (BORN) is mainly governed by the biaxial coherence length ξ b . Therefore, by varying ξ b , one can favour or inhibit the transient biaxial order. Recently, it has been demonstrated that ξ b can be controlled by suitable dopants and in this work we study the BORN transition threshold for liquid crystal mixtures made of the commercial nematic liquid crystal E7 and three different metallomesogens with board-like shape.

Polymer liquid crystalline composite for optical irradiation control

A composite material based on a liquid-crystalline gel has been prepared via the polymerization of diphenylolpropane dimethacrylate in the presence of the commercial nematic liquid crystal LC-1277 (94.3 wt %). Polymerization is performed under the action of UV irradiation. In the presence of the applied electric voltage, light scattering and birefringence of the samples are changed. The critical field of the electrooptical effect is found to be 2 V, the turn-on time of the effect is 9 ms (this time decreases with increasing voltage), and the turnoff time is 6 ms (independent of voltage). This material can be used in devices for optical irradiation control.

Transient capacitance study of switching in the nematic Freedericksz geometry

The transient capacitance has been measured for two commercial nematic liquid crystal materials, E7 and MDA01-2012, subjected to an amplitude modulated ac voltage wave form in the planar Freedericksz geometry. The wave form amplitude is modulated in a stepwise fashion between a value above the Freedericksz transition voltage VC [V. Freedericksz and V. Zolina, Trans. Faraday Soc. 29, 919 (1933)], causing switching, and a value below the threshold, where relaxation of the switched profile occurs. For both materials the time evolution of the dielectric permittivity during the switching and relaxation periods is accurately described by nematic continuum theory with a single viscosity provided that the switching voltage is a small multiple of VC.

Transient effects in electrolytic cells submitted to an external electric field

We analyse transient effects in an electrolytic cell submitted to an external voltage and determine the relaxation time of the redistribution of the ions and of the potential. We consider the case in which adsorption effects at the interface with the electrodes are present and show that the typical relaxation time, for applied voltage V 0⩽25 mV, is of the order of tens of seconds for commercial nematic liquid crystals. When V 0>25 mV the linearized analysis is no longer valid. In this case, the relaxation time depends on the applied voltage. Increasing V 0, but still remaining in the low amplitude limit, the relaxation time starts increasing. This indicates that the reduction of the actual field in the sample, for moderate values of V 0, plays an important role. For large values of V 0, the relaxation time is a decreasing function of V 0. This result is interpreted in terms of a simple model, according to which the ionic charge is localized in a surface layer whose thickness depends on the amplitude of the applied voltage.

Liquid Crystals Applications to R.F. and Microwave Tunable Components

We present an experimental procedure for dielectric characterization of liquid crystals in the frequency range 103 – 1010 Hz. This procedure is based an open-end strip line measuring cell and a 2D S.D.A. electromagnetic simulation software. Effective permittivities and tensor permittivity elements are determined for a commercial nematic liquid crystal. An application to an electrically tunable microwave phase-shifter is also presented. Transmission coefficient phase variations of about 0.5%/cm length are obtained per GHz showing the liquid crystals potentialities for such applications.